Method for making dimpled honeycomb sandwich



Dec. 16, 1969 M. l. KAZIMI 3,483,614

METHOD FOR MAKING DIMPLED HONEYCOMB SANDWICH Original Filed Dec. 14,1962 2 Sheets-Sheet l INVENTOR.

Mahmoud I. Kuzimi BY Dec. 16, 1969 M. 1. KAZIMI METHOD FOR MAKINGDIMPLED HONEYCOMB SANDWICH 2 Sheets-Sheet 2 Original Filed Dec. 14 1962Fig.5.

INVENTOR. Mohmdud l. Kczimi 3,483,614 METHOD FOR MAKING DIMPLEDHDNEYCOMB SANDWICH Mahmoud l. Kaziini, Berkeley, Calif, assignor toHexcel Products Inc., Berkeley, Calif.

Original application Dec. 14, 1962, Ser. No. 244,728, now Patent No.3,427,625. Divided and this application May 26, 1967, Ser. No. 668,261

Int. Cl. 1101p 11/00; H01q 13/00 US. Cl. 29-600 1 Claim ABSTRACT OF THEDISCLOSURE A method for making a dimpled face on a honeycomb sandwich bycreating a pressure differential across a face skin on a honeycomb core.The pressure differential is established by mechanical or pneumatictechniques.

This invention is a division of my copending applica tion for patentSer. No. 244,728, filed Dec. 14, 1962, now Patent No. 3,427,625.

This invention relates to a focusing reflector for radiatedelectromagnetic energy and more particularly to such reflector thatdiscriminates against interfering electromagnetic energy of shorter wavelength than the wave length of the radiated energy.

The preferred embodiment of the invention described herein and shown inthe drawings comprises a core formed of cellular honeycomb constructedof plural bonded ribbons, the edges of which ribbons are shaped todefine the contour of a concave surface such as a paraboloid. Thehoneycomb material has a cell size no greater than one quarter wavelength of the energy being reflected so that the surface of thehoneycomb material will be substantially opaque to such wave length andwill reflect the same. Signals having shorter wave lengths will passthrough the individual honeycomb cells and thus will not reflect to anantenna spaced relative to the concave surface.

Operation of paraboloid reflectors such as parabolic reflectors in anenvironment in which infra-red and shorter wave length radiation ispresent requires special precaution to prevent excessive heating of thereflector and to assure that the intensity of shorter wave lengthradiation does not exceed the intensity of the lower frequencyintelligence containing signals. The foregoing considerations havebecome particularly important in relation to the placement of antennasin space vehicles, because the intensity of infra-red and shorter wavelength radiation is greater in outer space since the radiation is notimpeded by the ionosphere. Reflectors having the foregoing electricalcharacteristics must also have sufficient rigidity that the reflectingsurface thereon maintains a desired shape. Additionally, an antenna ofthe type here under consideration should be lightweight, simple, andinexpensive to produce.

An object of this invention is to fulfil the foregoing desiderata.

Another object is to provide an antenna that has structural rigidity andhas a reflective surface with electrical continuity and frequencyselectivity so that higher interfering frequencies will not be reflectedby the device.

Still another object is to provide a method for expeditiously forming anantenna with the last named charactertics. The object includes providinga piece of honeycomb with a suitable contoured concave surface thereonand providing a thin sheet of conductive material, aflixing the skin tothe concave honeycomb surface, and creating a pressure differentialacross the thin sheet so that the thin sheet will be partially drawndown into the honeycomb cells. A myriad of small concavities or dimpleswill not have substantial effect on relatively longer wave length nitedStates Patent T 3,483,614 Patented Dec. 16, 1969 signals but will havesubstantial effect on shorter wave length signals. The latter will befocused at a point very near to the surface of the skin and will therebe dissipated, rather than being reflected to an antenna disposed infront of the concave surface to receive or transmit the longer wavelength signal.

Alternate embodiments of the invention can be constructed in which athin sheet of conductive material is affixed to the curved honeycombcore and is provided with a plurality of holes spaced from one anotherby an amount no greater than one quarter wave length of the signal beingreflected. The thin layer can also be formed of conductive mesh attachedto the edges of the honeycomb forming ribbons. These two modificationsprovide a path through the antenna for dissipation of thermal energycontained in infra-red and shorter interfering frequencies. Energy thatpasses through the openings in the conductive face will with highprobability also pass through the honeycomb cells. Thus, the reflectoris not heated to a temperature high enough to cause destruction thereofby the interfering short length waves.

These and other objects will be more apparent after referring to thefollowing specification and attached drawings in which:

FIG. 1 is a cross-sectional elevation view of a honeycomb core forming aparabolic reflector surface;

FIG. 2 is an enlarged view of a portion of the honeycomb core takensubstantially along line 22 of FIG. 1;

FIG. 3 is an enlarged cross-sectional elevational view showing anotherembodiment of the present invention;

FIG. 4 is an enlarged view of a portion of the antenna illustratingstill another embodiment of the invention;

FIG. 5 is a cross section view taken substantially along line 5-5 ofFIG. 4;

FIG. 6 is a plan view of yet another embodiment of the presentinvention; and

FIG. 7 is a cross section side view of the embodiment of FIG. 6.

Referring more particularly to the drawings, reference numeral 12denotes generally a honeycomb core formed by a plurality of flat ribbons14 bonded at recurring intervals as at 16 to form a plurality of cells17. The type honeycomb shown in FIG. 2 is hexagonal honeycomb but isshould be understood that any suitable shape cell structure can beprovided within the scope of the present invention. Core 12 ispreferably formed with cells wherein the maximum dimension betweenopposite surfaces of ribbon 14 is Within the range of one thirty-secondinch to one-quarter inch. Such range assures that the structure willhave the requisite rigidity and have the proper cell size for thedesired radio frequency selectivity. The upper edges 18 of ribbons 14are shaped to form a contour that defines a concave surface 20; thesurface is preferably a paraboloid to obtain optimum focusing but maytake other concave shapes as well. A reference point 22 in FIG. 1indicates the position of the focus of the paraboloid of curve surface20. A transmitting or receiving antenna is placed at focus 22 inaccordance with wellknown procedures.

Core 12 is supported upon a mast 24 that is attached to the reflectorremote from surface 20 of the core. A suitable antenna feed (not shown)can be mounted in a passage 26 formed axially with the major axis of theparaboloid surface.

In the embodiment depicted in FIG. 2, ribbons 14 are formed of aconductive material such as aluminum and are welded or brazed bysuitable material at their bond points 16. The weld points at bond 16electrically interconnect all areas of surface 20. Thus, the reflector,if constructed of one thirty-second inch honeycomb, will besubstantially opaque for frequencies up to about 10 cycles per second(3x10 in. wave length) and will reflect to focus 22 all signals havingfrequencies below that frequency. If one-quarter inch honeycomb is used,the paraboloid surface will appear substantially opaque to frequenciesup to about cycles per second (2 /2 10 m.). Energy having wave lengthsshorter than those referred to above will pass through cells 17 becausethe maximum dimension between opposing ribbons of a given cell exceedsone-quarter wave length. Because all ribbons 14 are electrically bondedat 16, electrical continuity will exist over the entire paraboloidsurface, and the entire surface will contribute to reflection of energyhaving relatively longer wave lengths.

Referring now to FIG. 3, core 12 is shown in greatly enlarged scale toemphasize an alternate embodiment of the invention. The edges 18 of thecore 12 are formed with a suitable concave shape, such as a paraboloidas in FIG. 1. To edges 18 is atfixed a conductive layer 28 by means ofan adhesive layer 30. In the present embodiment it is not necessary forribbons 14 or bond points 16 between adjacent ribbons to be electricallyconductive, because conductive sheet 28 provides electricalinterconnection between all areas of surface 20.

After a conductive sheet 28 is affixed to core 12, concavities ordimples 32 are formed. I prefer to form the concavities by evacuatingthe air from each cell 17 so that atmospheric pressure acts on the topof sheet 28 to cause partial attenuation or distortion thereof into thecells. In one reflector made according to my invention sheet 28 wasformed with a layer of 3 mil aluminum (.003 inch); a pressuredifferential across sheet 28 of approximately 2-3 atmospheres suitablyformed concavities 32. Any permanently extensile material can be used toform the concavities. The pressure diflerential formed by evacuatingcells 17 must be sufficient to exceed the elastic limit of thin sheet ofpermanently extensile material but insuflicient to rupture the thinsheet.

Each concavity 32 acts as a parabolic reflector for wave lengths thatare short relative to the distance between opposite points on theperimeter of each concavity. Because the perimeter is determined by theribbon edges 18, the frequency characteristics of the present embodimentare similar to those of the embodiment first described above.

The embodiments shown in FIGS. 1-3 have ribbons 14 parallel with theprincipal axis of the paraboloid of surface 20. Parallelism is notrequired; it is more important that the dimension between cell wallsalong surface 20 be chosen relative to the wave length of reflectedenergy and the interfering energy present to secure proper electricalcharacteristics. The openings or depressions presented to theelectro-magnetic radiation are thereby permitted to serve the intendedfunction.

An alternate method for forming depressions 32 in sheet 28 is to afiixthe skin to core 12 with adhesive layer 30, and then to move arelatively hard rubber roller over the surface. If the rubber of theroller is of suflicient hardness to attenuate thin conductive sheet 28,the desired dimples or concavities 32 will be formed.

A skin 34 can be attached to ribbon edges 18 which has a plurality ofrandomly positioned holes 36 therethrough (FIGS. 4 and 5). Holes 36 havea diameter no greater than the quarter wave lengths of the reflectedsignal, and are sufliciently numerous to pass shorter wave lengthenergy. Although removal of at least 80% of skin 34 by formation ofholes 36 therein affords adequate conveyance of shorter wave lengthenergy through the skin, up to 90% can be removed without excessivelyweakening the structure. Unwanted energy having a quarter wave lengthless than the diameter of holes 36 will pass through the holes to cells17, and will not interfere with the energy that is to be reflected byskin 34.

A conductive layer can be formed on the concave surface 20 of honeycombcore 12 by a conductive mesh 38 4 (FIG. 6). The mesh has pluralindividual conductors 38 that can be formed by expanded or stampedmetals or by intersecting wires. The conductors are both electricallyand mechanically bonded to one another at their intersecting points 40,so that electrical interconnection is attained between all areas of thesurface. Mechanical bonding of mesh 38 to ribbon ends 18 of core 12provides structural rigidity to the electrically continuous surface, sothat waves reflected therefrom are focused at point 22. The openingsbetween the conductors of mesh 38 are formed no larger than one-quarterwave length of the energy to be reflected by the device. Energy ofshorter wave lengths passes through the mesh and the honeycomb core andis not reflected to an antenna. Because mesh 38 forms the conductivesurface area, honeycomb core 12 in the embodiment of FIGS. 6 and 7 neednot be conductive material.

It is to be noted in connection with the embodiment of FIGS. 12 and 4-7that the cell forming side surfaces of ribbons 14 provide reflectingsurfaces for energy entering cells 17. Such energy is reflected back andforth between opposite cell walls until it is expelled from the rearopen end of cells 17. Consequently, the shorter wave length energy thatenters the cell will not interfere with the signal reflected to or froman antenna by surface 20.

Thus, I have provided a paraboloid reflector that is rigid, inexpensiveto produce and frequency selective. In addition, the structure hasextremely light weight. The novel method for manufacturing reflector canbe used to form reflectors of different shapes; and of differentfrequency discriminating characteristics.

While several embodiments of my invention have been shown and described,it will be apparent that other adaptations and modifications may be madewithout departing from the true spirit and Scope of the invention.

What is claimed is:

1. A method for forming a parabolic-shaped reflector for reflectingenergy of a predetermined wave length, comprising the steps of providinga piece of cellular core material generally defining a parabolic-shapedconcave surface, said honeycomb having cell dimensions no greater thanone-quarter of the length of said predetermined wave length, providing athin sheet of permanently extensile material of a size suflflcient tooverlay said parabolicshaped surface, applying a layer of adhesive toone face of said extensile material, bonding the extensile material tosaid parabolic-shaped surface, and thereafter creating a negativepressure differential through said honeycomb core and across saidextensile material of a magnitude sufficient to exceed the elastic limitof said material, but insufficient to rupture said material, so thatsaid material is distorted into the ends of the cells of said core toform a plurality of similar concave depressions whereby when saidreflector is employed for reflectively focusing radio frequency energy,signals having shorter wave lengths than said predetermined wave lengthwill not be reflected by the reflector.

References Cited UNITED STATES PATENTS 2,700,632 1/ 1955 Ackerlind29--455 2,799,318 7/1957 Blucher et al. 29-421 X 3,000,088 9/ 1961Melzer 29-42l 3,067,507 12/1962 Titus 29471.l 2,423,648 7/ 1947 Hansell343-84O 2,682,491 6/1954 Hahn 156197 X 3,097,982 7/1963 Stoner 15621lJOHN F. CAMPBELL, Primary Examiner D. C. REILEY, Assistant Examiner US.Cl. X.R.

2942l, 455; 156-l97; 16l-69; 343-840

